Since the Industrial Revolution in the 1700's, human activities, such as the burning of oil, coal and gas, and deforestation, have increased CO2 concentrations in the atmosphere. In 2005, global atmospheric concentrations of CO2 were 35% higher than they were before the Industrial Revolution. Conventional power plants are known as one of the largest sources of anthropogenic carbon dioxide emissions in the atmosphere. Additionally, several specialized industrial production processes, such as mineral or metal production and petroleum-based product generation, can also lead to CO2 emissions.
Carbon dioxide is one of the major greenhouse gases and the cause of global warming. The emissions from fossil fuel power plants are one of the largest sources of anthropogenic carbon dioxide emissions in the atmosphere. The carbon dioxide from the power plants can be separated from the sources via the following carbon dioxide capture processes: post-combustion, pre-combustion and oxyfuel combustion (Metz, B.; Davidson, O.; Coninck, H.; Loos, M.; Meyer, L. (Eds.) IPCC special report on carbon dioxide capture and storage. Cambridge University Press 2005). Among them, the pre-combustion process is considered as a feasible way to capture carbon dioxide in the clean coal gasification process or steam methane reforming (SMR) to produce hydrogen or electricity.
Sequestration of CO2 is becoming important for combating global climate change. Developed and developing countries are increasingly committed to reducing CO2 levels. To achieve CO2 target levels, these countries will have to enforce CO2 sequestration from concentrated CO2 sources, such as coal power plants.
Carbon dioxide in the flue gases from power plants can be captured by techniques such as absorption, adsorption, or reaction-based processes. Of all the methods, the calcium oxide based reaction process for carbon dioxide capture from the effluent gas (Shimizu, T.; Hirama, T.; Hosoda, H.; Kitano, K.; Inagaki, M.; Tejima, K. A twin fluid-bed reactor for removal of CO2 from combustion processes. Trans. IChemE 1999, 77 (Part A), 62-68) looks very promising considering the operating temperature and pressure, capture capacity, the low carbon dioxide concentration (5-30%) in the effluent, and regeneration of pure carbon dioxide through the calcination reaction (Gupta, H.; Fan, L-S. Carbonation-calcination cycle using high reactivity calcium oxide for carbon dioxide separation from flue gas. Ind. Eng. Chem. Res. 2002, 41, 4035-4042).
Calcium oxide is an effective carbon dioxide absorbent, but the cyclic lifetime and durability of the absorbent are key issues for its practicability. Pore plugging and sintering of particles have been identified as the major causes of the degradation of the absorption capacity in cyclic operation. The carbonation between calcium oxide and carbon dioxide and calcination reactions are described as follows:Carbonation:CaO(s)+CO2(g)→CaCO3(s), H973K=−169.7 kJ/mol (exothermic)  (1)Calcination:CaCO3(s)→CaO(s)+CO2(g), H1173K=166.3 kJ/mol (endothermic)  (2)
Carbon dioxide reacts with calcium oxide to form calcium carbonate in the carbonation reaction and the calcium oxide is regenerated and pure carbon dioxide can be obtained through the calcination reaction. This calcium oxide absorbent has been investigated to improve the process efficiency as well as trap the carbon dioxide in biomass or coal gasification process (Mahishi, M. R.; Goswami, D. Y. An experimental study of hydrogen production by gasification of biomass in the presence of a CO2 sorbent. International Journal of Hydrogen Energy 2007, 32, 2803-2908; Xu, G.; et al. Distinctive effects of CaO additive on atmospheric gasification of biomass at different temperatures Ind. Eng. Chem. Res. 2005, 44, 5864-5868; Hanaoka, T.; et al. Hydrogen production from woody biomass by steam gasification using a CO2 sorbent. Biomass and Bioenergy 2005, 28, 63-68; Lin, S.; et al.; Hydrogen production from coal by separating carbon dioxide during gasification. Fuel 2002, 81, 2079-2085; Feng, B.; et al. Screening of CO2 adsorbing materials for zero emission power generation systems. Energy & Fuels 2007, 21, 426-434; Slowinski, G. Some technical issues of zero-emission coal technology. International Journal of Hydrogen Energy 2006, 31, 1091-1102; Feng, B.; et al. Overcoming the problem of loss-in-capacity of calcium oxide in CO2 capture. Energy & Fuels 2006, 20, 2417-2420). It was reported that the hydrogen yield from pyrolysis of the mixture of coal and calcium oxide, compared to conventional coal pyrolysis, was more than five times (Lin, S.; et al.; Hydrogen production from coal by separating carbon dioxide during gasification. Fuel 2002, 81, 2079-2085).
However, substantial volume changes between carbonate (36.9 cm3/mol) and oxide forms (16.9 cm3/mol) are induced by these gas-solid reactions (Stanmore, B. R.; Gilot, P. Review-calcination and carbonation of limestone during thermal cycling for CO2 sequestration. Fuel Processing Technology 2005, 86, 1707-1743). These structural and thermal stresses caused by the cyclic carbonation-calcination reaction lead to the loss in active surface area, pore plugging and sintering of the particles in the absorbent. The degradation of the CO2 absorption capacity during the cyclic operation of the absorbent (calcium oxide), which is caused by the loss in surface area due to pore plugging and sintering of particles, must be overcome for the process to be practical (Barker, R. The reversibility of the reaction CaCO3=CaO+CO2. J. Appl. Chem. Biotechnol 1973, 23, 733-742; Borgwardt, R. H. Calcium oxide sintering in atmospheres containing water and carbon dioxide. Ind. Eng. Chem. Res. 1989, 28, 493-500). Various forms of calcium oxide have been investigated to improve the durability of cyclic performance for capturing carbon dioxide, such as dolomite (CaCO3.MgCO3) (Curran, G. P.; et al. Carbon dioxide-acceptor gasification process: studies of acceptor properties. Adv. Chem. Ser. 1967, 69, 141-165; Dobner, S.; et al. Cyclic calcinations and recarbonation of calcined dolomite. Ind. Eng. Chem., Process Des. Dev. 1977, 16, 479-486; Silaban, A.; et al. Characteristics of the reversible reaction between CO2(g) and calcined dolomite. Chem. Eng. Comm 1996, 146, 149-162), calcium oxide dispersed in porous inert calcium titanate (CaTiO3) matrix (Aihara, M.; et al. Development of porous solid reactant for thermal-energy storage and temperature upgrade using carbonation/decarbonation reaction. Applied Energy 2001, 69, 225-238), impregnated in porous alumina granules (Feng, B.; et al. Overcoming the problem of loss-in-capacity of calcium oxide in CO2 capture. Energy & Fuels 2006, 20, 2417-2420), or mixed with mayenite (Ca12Al14O33) (Li, Z.-S.; et al. Synthesis, experimental studies, and analysis of a new calcium-based carbon dioxide absorbent. Energy & Fuels 2005, 19, 1447-1452; Li, Z.-S.; et al. Effect of preparation temperature on cyclic CO2 capture and multiple carbonation-calcination cycles for a new ca-based CO2 sorbent. Ind. Eng. Chem. Res. 2006, 45, 1911-1917) or nano-sized alumina (Al2O3) particles (Wu, S. F.; et al. Properties of a nano CaO/Al2O3CO2 sorbent. Ind. Eng. Chem. Res. 2008, 47, 180-184), and core-in-shell catalysis/sorbent (Satrio, J. A.; et al. Application of combined catalyst/sorbent on hydrogen generation from biomass gasification. AIChE Annual Meeting Conference Proceedings, Nov. 7-12, 2004, Austin, Tex.; Satrio, J. A.; et al. Development of a novel combined catalyst and sorbent for hydrocarbon reforming. Ind. Eng. Chem. Res. 2005, 44, 3901-3911; Satrio, J. A.; et al. A combined catalysis and sorbent for enhancing hydrogen production from coal or biomass. Energy & Fuels 2007, 21, 322-326) have been introduced and investigated to improve the cyclic performance of the absorbents for carbon dioxide capture. Most of these attempts showed better cyclic performance than pure calcium oxide thanks to the inert materials, but those bring other drawbacks, complexity in preparation, high cost for the synthesis and low content of calcium oxide in inert materials.
Conventional steam methane reforming (SMR) can be enhanced via in situ carbon dioxide capture using sorbents. The process is known as sorption-enhanced steam methane reforming process (SE-SMR). Hydrogen yield was increased along with carbon dioxide capture through the carbonation reaction (3) since the equilibrium shifts to the right by removing carbon dioxide by the calcium oxide based sorbents in a water-gas shift reaction shown below (Han, C.; Harrison, D. P. Simultaneous shift and carbon dioxide separation for the direct production of hydrogen. Chem. Eng. Sci. 1994, 49, 5875-5883; Balasubramanian, B.; Ortiz, A. L.; Kaytakoglu, S.; Harrison, D. P. Hydrogen from methane in a single-step process. Chem. Eng. Sci. 1999, 54, 3543-3552; Li, Z.-S.; Cai, N.-S.; Yang, J.-B. Continuous production of hydrogen from sorption-enhanced steam methane reforming in two parallel fixed-bed reactors operated in a cyclic manner. Ind. Eng. Chem. Res. 2006, 45, 8788-8793). The conventional coal gasification process and steam methane reforming consists of the following a reformation and a water-gas shift reaction reformationCHx+H2O←→CO+yH2  (3)
(coal gasification: x=0, y=1, SMR: x=4, y=3)
water-gas shift reactionH2O+CO←→CO2+H2  (4)
In addition to its use in carbon dioxide capture for biomass or coal gasification process, calcium oxide has also been considered as a feasible candidate for efficiency improvement (Feng, B.; et al. Screening of CO2 adsorbing materials for zero emission power generation systems. Energy & Fuels 2007, 21, 426-434; Hanaoka, T.; et al. Hydrogen production from woody biomass by steam gasification using a CO2 sorbent. Biomass and Bioenergy 2005, 28, 63-68). Hydrogen yield from these processes can be increased significantly using absorbents to react with or absorb carbon dioxide during gasification since the forward equilibrium shift would occur by removing carbon dioxide in a water-gas shift reaction (Mahishi, M. R.; Goswami, D. Y. An experimental study of hydrogen production by gasification of biomass in the presence of a CO2 sorbent. International Journal of Hydrogen Energy 2007, 32, 2803-2908; Balasubramanian, B.; et al. Hydrogen from methane in a single-step process. Chem Eng Sci 1999, 54, 3543-3552; Lin, S.-Y.; et al. Developing an innovative method, HyPr-RING, to produce hydrogen from hydrocarbons. Energy Conservation and Management 2002, 43, 1283-1290). Various absorbents has been introduced and studied, but calcium oxide based absorbents seem very promising in consideration of operating temperature, pressure and capture capacity (Gupta, H.; Fan, L. Carbonation-calcination cycle using high reactivity calcium oxide for carbon dioxide separation from flue gas. Ind. Eng. Chem. Res. 2002, 41, 4035-4042). The zero-emission coal (ZEC) process using calcium oxide for carbon dioxide capture also had higher hydrogen yield. The process is very attractive in that electricity can be generated with high efficiency and without emission of carbon dioxide assuming cyclic stability of the CaO/CaCO3 bed (Slowinski, G. Some technical issues of zero-emission coal technology. International Journal of Hydrogen Energy 2006, 31, 1091-1102).
Compounds such as calcium oxide or calcium hydroxide have been considered as promising candidates for carbon sequestration, absorbing carbon dioxide to form calcium carbonate. The original compounds can be regenerated by desorbing carbon dioxide under the proper conditions. However, one key problem relates to large volume changes during the carbonation/decarbonation reactions. This problem potentially severely limits the cyclic repeatability of this process. The published literature currently shows no more than one or two cycles can be completed before capacity is reduced drastically. To create a practical calcium oxide or calcium hydroxide scrubber, the degradation of the absorption capacity in cyclic operation, caused by pore plugging and sintering of particles, must be addressed.